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Date:   Thu, 17 Jan 2019 17:35:13 +0000
From:   Mel Gorman <>
To:     Vlastimil Babka <>
Cc:     Linux-MM <>,
        David Rientjes <>,
        Andrea Arcangeli <>,,, Andrew Morton <>,
        Linux List Kernel Mailing <>
Subject: Re: [PATCH 16/25] mm, compaction: Check early for huge pages
 encountered by the migration scanner

On Thu, Jan 17, 2019 at 06:01:18PM +0100, Vlastimil Babka wrote:
> On 1/4/19 1:50 PM, Mel Gorman wrote:
> > When scanning for sources or targets, PageCompound is checked for huge
> > pages as they can be skipped quickly but it happens relatively late after
> > a lot of setup and checking. This patch short-cuts the check to make it
> > earlier. It might still change when the lock is acquired but this has
> > less overhead overall. The free scanner advances but the migration scanner
> > does not. Typically the free scanner encounters more movable blocks that
> > change state over the lifetime of the system and also tends to scan more
> > aggressively as it's actively filling its portion of the physical address
> > space with data. This could change in the future but for the moment,
> > this worked better in practice and incurred fewer scan restarts.
> > 
> > The impact on latency and allocation success rates is marginal but the
> > free scan rates are reduced by 32% and system CPU usage is reduced by
> > 2.6%. The 2-socket results are not materially different.
> Hmm, interesting that adjusting migrate scanner affected free scanner. Oh well.

Russian Roulette again. The exact scan rates depend on the system state
which are non-deterministic.  It's not until very late in the series that
they stabilise somewhat. In fact, during the development of the series,
I had to reorder patches multiple times when a corner case was dealt with
to avoid 1 in every 3-6 runs having crazy insane scan rates. The final
ordering was based on *relative* stability.

> > Signed-off-by: Mel Gorman <>
> Acked-by: Vlastimil Babka <>
> Nit below.

Nit fixed.

Mel Gorman

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